A powered surgical instrument, a surgical tool, used with some frequency is the powered surgical saw. This type of instrument is used to remove tissue, including bone and cartilage. Attached to the saw is a saw blade. A drive assembly internal to the saw reciprocates the blade in a back and forth motion. Often the saw includes a moving head. The head is the component of the saw to which the blade is mounted. Some blades are constructed to pivot back and forth, oscillate, in the plane in which the blade is oriented. This type of blade is referred to as a sagittal saw blade. A sagittal saw blade is provided with teeth that extend forward from the distal end of the blade body.
Many sagittal saws and their complementary blades are designed so that the blade extends distally forward of the blade head. One such assembly is disclosed in the Applicant's U.S. Pat. No. 8,100,912/PCT Pub. No. WO 2007/011542, the contents of which are incorporated herein by reference. This type of saw and blade are used to remove a section of bone. This is perhaps the most common type of sagittal saw.
A surgical saw includes an assembly for removably holding the blade to the saw. This is because the blade is often the single use component of the combined saw and blade assembly. One reason the blade is used once is that upon use of the blade, the teeth are immediately dulled. Owing to the economics, it is often more cost effective to use a new blade with each patient than go to the expense of sterilizing and resharpening a previously used blade.
A surgical sagittal saw is typically formed so that the head has a slot. The slot is the void space dimensioned to receive the proximal end of the blade. Often, the proximal end of the blade is provided with one or more openings. Each opening is dimensioned to receive a pin that is moveably mounted to the saw head. The seating of the pin in the blade opening releasably holds the blade to the head.
It is common practice to collectively dimension the saw head and blade so the slot facilitates the close slip fitting of the blade in the slot. This dimensioning facilitates the relatively easy insertion of the blade into the saw head and removal of the blade from the saw head. A result of this component dimensioning is that within the slot, there is small clearance between the blade and the interior surfaces of the saw head that define the slot. This means that within the slot the blade has space to move.
Owing to this tendency of the blade to move, the back-and-forth movement of the blade is not always in phase with the back-and-forth movement of the saw head. This out of phase movement occurs because when the saw head reverses direction, owing to the blade having a momentum in the opposite direction, the blade continues to move in the first direction. Thus, there may be times in the movement of the saw head and blade where these two components move in the opposed directions. This can result in the blade striking an adjacent surface of the saw head. This action is sometimes referred to as blade slap. A result of blades continually slapping against the saw head is that the material forming the head can fatigue. This component fatigue can result in the fracturing of the saw head. Once such a fracture occurs, at a minimum, it is necessary to replace the saw head.
The failure of the blade to move completely in unison with the saw head can even result in problems even when the blade does not strike the saw head. The lagging movement of the blade relative to the saw head is sometimes referred to as backlash. As a result of this movement each tooth of the blade may not, in a single phase of a single oscillatory cycle, move in an arc of sufficient length. More particularly, for a blade to efficiently function, during a single phase of movement, a tooth of the blade should move at least as far as the starting position of the adjacent tooth at the start of the phase. For example, when a blade sweeps right, a tooth should move to the right a sufficient distance so that, at the end of the sweep, the tooth will have moved to at least the location at which the adjacent tooth to the right of the blade was located at the start of the sweep. When a tooth engages in this degree of arcuate movement, there is high likelihood that, in the single sweep the tooth will have sheared away the bone located between that tooth and the adjacent right located tooth. This removal of all the bone between the teeth is what facilitates the efficient formation of the cut.
The problem arises because, owing to the backlash, the saw blade and by extension the teeth of the saw, in a single phase of movement, engages in an arcuate movement that is less than the arcuate movement of the saw head. In some situations that means that in the single phase of movement, a blade will not sweep to the location at which the adjacent tooth was located at the start of the sweep. When this event occurs, not all the bone between the teeth are sheared, cut away. This can reduce the efficiency of the cutting process.
In addition to the blade moving side to side relative to the saw head, a blade may be able to move up and down relative the head slot in which the blade is seated. This movement is sometimes referred to as the out of plane oscillation of the blade. Alternatively, this movement is sometimes referred to as blade whip. This out of plane movement of the blade relative to the saw can adversely affect the precision of the cut formed by the blade. This movement can also stress both the saw head and the proximal end of the blade, the portion of the blade seated in the saw head slot. The stressing of the saw head can result in the fracturing of the saw head. The stressing of the proximal end of the blade can result in the deformation of the blade. This deformation can also reduce the precision of the cut made by the blade.
Another disadvantage of this movement of the blade relative to the saw head is that it can result in the blade moving to a less than optimal position for the procedure being performed. When a sagittal saw is used to remove a large section of the bone such as a portion of the knee, the blade is often placed in a resection guide. This instrument is a block that is affixed to the bone adjacent where the cut is to be formed. The block is formed with one or more slots. The slots serve as guide paths through which the saw blade is inserted. By cutting the bone along the guide paths defined by the slots, the bone left after the cut will have the desired shape. This precision shaping of the bone ensures the proper fitting of an orthopedic implant to the bone. Owing to the flexure of the blade when fitted in one of these slots, the blade can gall, wear the material that defines the slots. This can result in the shape of the slot deforming from the shape needed to ensure that a cut formed based on the shape of the slot has the desired shape. Once this deformation of the resection guide occurs, the guide is no longer useful.
There is therefore a desire to provide a surgical sagittal saw and complementary blade that are constructed so that, when the saw and blade are actuated, the saw head and blade move as a single rigid structure. One means to ensure the saw head and blade move as a single rigid body is to provide a clamping assembly that, when set, applies an appreciable amount of force to the blade to hold the blade to the saw head. This typically means that the individual charged with blade insertion and blade removal needs to apply a significant amount of force in order to reset and release the clamping assembly. Requiring the individual responsible to perform these tasks to apply these forces can complicate the process of inserting and removing the blade. Requiring the individual to apply these forces can also slow the processes associated with both inserting and removing the blade. This is especially true if the individual has limited arm and hand strength. Further, if these forces are not properly applied, especially the force required to set the clamping assembly, the blade may not be fully locked to the saw head. When the saw is actuated this could result in a clearly undesirable event, the blade working free from the saw.